This invention relates generally to the testing of semiconductors, and more particularly to an internal guardband for such semiconductor testing.
Semiconductor technology pervades most electronic devices today. Computers, televisions, videocassette recorders, cameras, etc., all use semiconductor integrated circuits to varying degrees. For example, the typical computer includes microprocessors and dedicated controller integrated circuits (i.e., video controllers, audio controllers, etc.), as well as memory, such as dynamic random-access memory. The design and testing of semiconductors, therefore, is a crucial consideration of the design of almost any electronic device.
In one example of testing a semiconductor circuit, the circuit has a data strobe line that is asserted a predetermined amount of time after the beginning of a clock cycle. This is accomplished to test whether the circuit is operating correctly. For example, if the circuit is not operating correctly, asserting the data strobe line after a predetermined amount of time after the beginning of a clock cycle (defined as the access time) will produce the wrong effect or no effect at all. That is, the circuit will not output any data, or will not output the correct data.
The assertion of the data strobe line is usually performed by a testing device. Because of variance among different testing devices, the data strobe line is asserted a predetermined amount of time after the beginning of a clock cycle that is less than the amount of time governed by the specification for the semiconductor circuit. This difference between the amount of time actually waited by the testing device before asserting the data strobe line and the amount of time as governed by the circuit""s specification is known as the guardband. (Those of ordinary skill within the art will appreciate that guardband is not definitionally limited to this difference, however, but rather that this difference is only one instance of guardband.) Providing a guardband during the testing of the circuit ensures that the circuit is operating correctly, allowing for variances among different testing devices, as well for allowable manufacturing tolerances within the circuit itself.
The prior art provides for guardband typically by the testing device itself. For example, if a circuit specification allows for fifteen nanosecond access time, the semiconductor circuit tester can provide for a four nanosecond guardband by testing the circuit at an eleven nanosecond access time. The difference between the specified access time and the guardband thus indicates to the tester the necessary access time to be dialed in to the testing device.
However, with the increased speed of semiconductor devices such as semiconductor memories, the resultant decreased clock cycles, access times, and hold times (the latter defined as the length of time that a line is asserted after waiting the appropriate access time) may render it difficult or impossible to provide a guardband by the testing device itself. For example, as those of ordinary skill within the art can appreciate, if a hold time is specified as one nanosecond, it is very difficult to provide for two nanoseconds of guardband in the testing of a semiconductor circuit.
This puts the semiconductor circuit tester in a compromising position. The tester, to allow for variances among different testing devices, should still provide a guardband when testing a semiconductor circuit. However, the prior art manner by which guardband is typically afforded, for example, by appropriately decreasing the access time as tested by a testing device, may not prove workable. Therefore, there is a need for providing a guardband in a manner that is not dependent on a testing device. That is, there is a need for ensuring guardband where guardband is not or cannot be provided by a testing device. Such a guardband should in particular be able to be provided even during the testing of very fast semiconductor devices.
The above-mentioned shortcomings, disadvantages and problems are addressed by the present invention, which will be understood by reading and studying the following specification. One aspect of the invention is a semiconductor circuit having two paths. The first path is a standard path, used for normal operation of the circuit. The second path is a test path, used for testing of the circuit. The second, test path adds delay as compared to the first, standard path.
During the testing of the semiconductor circuit, the test path is used by a testing device, instead of the standard path. The delay added by the test path acts as a guardband for the semiconductor circuit. Thus, for example, rather than decreasing the access time for the circuit as tested by the testing device, to have the testing device itself provide the guardband, the use of the test path by itself inherently provides the guardband. This means that the access time as governed by the specification for the circuit can be dialed in to the testing device, instead of the specified access time minus a desired guardband. The guardband is therefore internal to the semiconductor circuit itself, instead of being provided by an external testing device.
Providing for this internal guardband extends to the invention advantages not found in the prior art. Internal guardband is not dependent on a given testing device. Therefore, in instances where the testing device cannot provide a desired guardband, a guardband still exists because of the test path within the semiconductor circuit itself. This means that very fast semiconductor devices can be tested with a guardband, where testing devices would not otherwise be able to provide the guardband.
The present invention includes semiconductor circuits, semiconductor devices, guardband test circuits, and methods of varying scope. In addition to the aspects and advantages of the present invention described in this summary, further aspects and advantages of the invention will become apparent by reference to the drawings and by reading the detailed description that follows.